title: A low Z linac and flat panel imager: comparison with the conventional imaging approach.
abstract: Experimental and Monte Carlo simulations were conducted for an Elekta Ltd Precise Treatment System linac fitted with a low Z insert of sufficient thickness to remove all primary electrons. A variety of amorphous silicon based panels employing different scintillators were modelled to determine their response to a variety of x-ray spectra and produce an optimized portal imaging system. This study has shown that in a low Z configuration the vast majority of x-rays are produced in the nickel electron window, and with a combination of a carbon insert and caesium iodide based XVI-panel, significant improvement in the object contrast was achieved. For thin, head and neck-type geometries, contrast is 4.62 times greater for 1.6 cm bone in 5.8 cm water than the standard 6 MV/iViewGT system. For thicker, pelvis-type geometries contrast increases by a factor of 1.3 for 1.6 cm of bone in 25.8 cm water. To obtain images with the same signal-to-noise ratio as the 6 MV/iViewGT system, dose reductions of a factor of 15 and 4.2 are possible for 5.8 cm and 25.8 cm phantoms respectively. This design has the advantage of being easily implemented on a standard linac and provides a portal image directly from the therapy beam aperture.
authors.affiliation: Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Downs Road, Sutton, Surrey, UK.
authors.forename: D A
authors.initials: DA
authors.lastname: Roberts
authors.forename: V N
authors.initials: VN
authors.lastname: Hansen
authors.forename: A C
authors.initials: AC
authors.lastname: Niven
authors.forename: M G
authors.initials: MG
authors.lastname: Thompson
authors.forename: J
authors.initials: J
authors.lastname: Seco
authors.forename: P M
authors.initials: PM
authors.lastname: Evans
mesh_terms: D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D017785:Photons; D011829:Radiation Dosage; D011880:Radiotherapy Planning, Computer-Assisted